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13 Trace Evidence I: Hairs and Fibers

© Bettmann/CORBIS All Rights Reserved

LEARNING OBJECTIVES

After studying this chapter, you should be able to:

• Recognize and understand the cuticle, cortex, and medulla areas of hair. • List the three phases of hair growth. • Appreciate the distinction between animal and human hairs. • List hair features that are useful for microscopic comparisons of human hairs. • Explain the proper collection of forensic hair evidence. • Describe and understand the role of DNA typing in hair comparisons. • Understand the differences between natural and manufactured fibers. • List the properties of fibers that are most useful for forensic comparisons. • Describe the proper collection of fiber evidence.

JEFFREY MACDONALD: FATAL VISION

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The grisly murder scene that confronted police on February 17, 1970, is one that would not be wiped from memory. Summoned to the Fort Bragg residence of Captain Jeffrey MacDonald, a physician, police found the bludgeoned body of MacDonald’s wife. She had been repeatedly knifed, and her face was smashed to a pulp. MacDonald’s two children, ages 2 and 5, had been brutally and repeatedly knifed and battered to death.

Suspicion quickly fell on MacDonald. To the eyes of investigators, the murder scene had a staged appearance. MacDonald described a frantic effort to subdue four intruders who had slashed at him with an ice pick. However, the confrontation left MacDonald with minor wounds and no apparent defensive wounds on his arms. MacDonald then described how he had covered his slashed wife with his blue pajama top. Interestingly, when the body was removed, blue threads were observed under the body. In fact, blue threads matching the pajama top turned up throughout the house—nineteen in one child’s bedroom, including one beneath her fingernail, and two in the other child’s bedroom. Eighty-one blue fibers were recovered from the master bedroom, and two were located on a bloodstained piece of wood outside the house.

Forensic examination showed that the forty-eight ice pick holes in the pajama top were smooth and cylindrical, a sign that the top was stationary when it was slashed. Also, folding the pajama top demonstrated that the forty-eight holes actually could have been made by twenty-one thrusts of an ice pick. This coincided with the number of wounds that MacDonald’s wife sustained. As described in the book Fatal Vision, which chronicles the murder investigation, when MacDonald was confronted with adulterous conduct, he replied, “You guys are more thorough than I thought.” MacDonald is currently serving three consecutive life sentences.

The trace evidence transferred between individuals and objects during the commission of a crime, if recovered, often corroborates other evidence developed during the course of an investigation. Although in most cases physical evidence cannot by itself positively identify a suspect, laboratory examination may narrow the origin of such evidence to a group that includes the suspect. Using many of the instruments and techniques we have already examined, the crime laboratory has developed a variety of procedures for comparing and tracing the origins of physical evidence. This chapter will focus on the value of hairs and fibers as physical evidence.

Forensic Examination of Hair

Hair is encountered as physical evidence in a wide variety of crimes. However, any review of the forensic aspects of hair examination must start with the observation that it is not yet possible to individualize a human hair to any single head or body through its morphology, or structural characteristics. Over the years, criminalists have tried to isolate the physical and chemical properties of hair that could serve as individual characteristics of identity. Partial success has finally been achieved by isolating and characterizing the DNA present in hair.

The importance of hair as physical evidence cannot be overemphasized. Its removal from the body often denotes physical contact between a victim and perpetrator and hence a crime of a serious or violent nature. When hair is properly collected at the crime scene and submitted to the laboratory along with enough standard/reference samples, it can provide strong corroborative evidence for placing an individual at a crime site. The first step in the forensic examination of hair logically starts with its color and structure (i.e., morphology) and, if warranted, progresses to the more detailed DNA extraction, isolation, and characterization.

MORPHOLOGY OF HAIR

Hair is an appendage of the skin that grows out of an organ known as the hair follicle. The length of a hair extends from its root, or bulb, which is embedded in the follicle, continues into the shaft, and terminates at the tip. The shaft, which is composed of three layers—the cuticle, cortex, and medulla—is most intensely examined by the forensic

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scientist (see Figure 13-1).

cuticle

The scale structure covering the exterior of the hair.

cortex

The main body of the hair shaft.

medulla

A cellular column running through the center of the hair.

CUTICLE

Two features that make hair a good subject for establishing individual identity are its resistance to chemical decomposition and its ability to retain structural features over a long period of time. Much of this resistance and stability is attributed to the cuticle, a scale structure covering the exterior of the hair. The cuticle is formed by overlapping scales that always point toward the tip end of each hair. The scales form from specialized cells that have hardened (i.e., keratinized) and flattened in progressing from the follicle. There are three basic patterns that describe the appearance of the cuticle: cornal, spinous, and imbricate (see Figure 13-2).

Although the scale pattern is not a useful characteristic for individualizing human hair, the variety of patterns formed by animal hair makes it an important feature for species identification. Figure 13-3 shows the scale patterns of some animal hairs and of a human hair as viewed with a scanning electron microscope. Another method of studying the scale pattern of hair is to make a cast of its surface. This is done by embedding the hair in a soft medium, such as clear nail polish or softened vinyl. When the medium has hardened, the hair is removed, leaving a clear, distinct impression of the hair’s cuticle, ideal for examination with a compound microscope.

FIGURE 13-1 A cross-section of skin showing hair growing out of a tubelike structure called the follicle.

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FIGURE 13-2 (a) The coronal, or crownlike, scale pattern resembles a stack of paper cups. (b) Spinous or petal-like scales are triangular in shape and protrude from the hair shaft. (c) The imbricate, or flattened-scale, type consists of overlapping scales with narrow margins. Richard Saferstein, Ph.D.

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CORTEX

Contained within the protective layer of the cuticle is the cortex, the main body of the hair shaft. The cortex is made up of spindle-shaped cortical cells aligned in a regular array, parallel to the length of the hair. The cortex derives its major forensic importance from the fact that it is embedded with the pigment granules that give hair its color. The color, shape, and distribution of these granules provide important points of comparison among the hairs of different individuals.

FIGURE 13-3 Scale patterns of various types of hair: (a) human head hair

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(600×), (b) dog (1350×), (c) deer (120×), (d) rabbit (300×), (e) cat (2000×), and (f) horse (450×).

The structural features of the cortex are examined microscopically after the hair has been mounted in a liquid medium with a refractive index close to that of the hair. Under these conditions, the amount of light reflected off the hair’s surface is minimized, and the amount of light penetrating the hair is optimized.

MEDULLA

The medulla is a collection of cells that looks like a central canal running through a hair. In many animals, this canal is a predominant feature, occupying more than half of the hair’s diameter. The medullary index measures the diameter of

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the medulla relative to the diameter of the hair shaft and is normally expressed as a fraction. For humans, the index is generally less than one-third; for most other animals, the index is one-half or greater.

The presence and appearance of the medulla vary from individual to individual and even among the hairs of a given individual. Not all hairs have medullae, and when they do exist, the degree of medullation can vary. In this respect, medullae may be classified as being either continuous, interrupted, fragmented, or absent (see Figure 13-4). Human head hairs generally exhibit no medullae or have fragmented ones; they rarely show continuous medullation. One noted exception is in people of the Mongoloid race, who usually have head hairs with continuous medullae. Also, most animals have medullae that are either continuous or interrupted.

FIGURE 13-4 Medulla patterns.

FIGURE 13-5 Medulla patterns for various types of hair: (a) human head hair (400×), (b) dog (400×), (c) deer (500×), (d) rabbit (450×), (e) cat (400×), and (f) mouse (500×).

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Another interesting feature of the medulla is its shape. Humans, as well as many animals, have medullae that give a nearly cylindrical appearance. Other animals exhibit medullae that have a patterned shape. For example, the medulla of a cat can best be described as resembling a string of pearls, whereas members of the deer family show a medullary structure consisting of spherical cells occupying the entire hair shaft. Figure 13-5 illustrates medullary sizes and forms for a number of common animal hairs and a human head hair.

A searchable database on CD-ROM of the thirty-five most common animal hairs encountered in forensic casework is

commercially available.1 This database allows an examiner to rapidly search for animal hairs based on scale patterns and/or medulla type using a PC. A typical screen presentation arising from such a data search is shown in Figure 13-6.

ROOT

The root and other surrounding cells within the hair follicle provide the tools necessary to produce hair and continue its growth. Human head hair grows in three developmental stages, and the shape and size of the hair root is determined by

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the hair’s current growth phase. The three phases of hair growth are the anagen, catagen, and telogen phases.

anagen phase

The initial growth phase during which the hair follicle actively produces hair.

catagen phase

A transition stage between the anagen and telogen phases of hair growth.

telogen phase

The final growth phase in which hair naturally falls out of the skin.

FIGURE 13-6 Information on rabbit hair contained within the Forensic Animal Hair Atlas.

Courtesy RJ Lee Group, Inc. Monroeville, PA

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FIGURE 13-7 Hair roots in the (a) anagen phase, (b) catagen phase, and (c) telogen phase (100×).

Courtesy Charles A. Linch

In the anagen phase (the initial growth phase), which may last up to six years, the root is attached to the follicle for continued growth, giving the root bulb a flame-shaped appearance (Figure 13-7[a]). When pulled from the root, some hairs in the anagen phase have a follicular tag. With the advent of DNA analysis, this follicular tag is important for individualizing hair.

follicular tag

A translucent piece of tissue surrounding the hair’s shaft near the root that contains the richest source of DNA associated with hair.

Hair continues to grow, but at a decreasing rate, during the catagen phase, which can last anywhere from two to three weeks. In the catagen phase, roots typically take on an elongated appearance (Figure 13-7[b]) as the root bulb shrinks and is pushed out of the hair follicle. Once hair growth ends, the telogen phase begins and the root takes on a club-shaped appearance (Figure 13-7[c]). Over two to six months, the hair is pushed out of the follicle, causing the hair to be naturally shed.

IDENTIFICATION AND COMPARISON OF HAIR

Most often the prime purpose for examining hair evidence in a crime laboratory is either to establish whether the hair is human or animal in origin or to determine whether human hair retrieved at a crime scene compares with hair from a particular individual. A careful microscopic examination of hair reveals morphological features that can distinguish human hair from animal hair. The hair of various animals also differs enough in structure that the examiner can often identify the species. Before reaching such a conclusion, however, the examiner must have access to a comprehensive collection of reference standards and the accumulated experience of hundreds of prior hair examinations. Scale structure, medullary index, and medullary shape are particularly important in hair identification.

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The most common request when hair is used as forensic evidence is to determine whether hair recovered at the crime scene compares to hair removed from a suspect. In most cases, such a comparison relates to hair obtained from the scalp or pubic area. Ultimately, the evidential value of the comparison depends on the degree of probability with which the examiner can associate the hair in question with a particular individual.

FACTORS IN COMPARISON OF HAIR

Although animal hair normally can be distinguished from human hair with little difficulty, human hair comparisons must be undertaken with extreme caution. Hair tends to exhibit variable morphological characteristics, not only from one person to another but also within a single individual. In comparing hair, the criminalist is particularly interested in matching color, length, and diameter. Other important features are the presence or absence of a medulla and the distribution, shape, and color intensity of the pigment granules in the cortex. A microscopic examination may also distinguish dyed or bleached hair from natural hair. A dyed color is often present in the cuticle as well as throughout the cortex. Bleaching, on the other hand, tends to remove pigment from the hair and gives it a yellowish tint.

If hair has grown since it was last bleached or dyed, the natural-end portion will be quite distinct in color. An estimate of the time since dyeing or bleaching can be made because hair grows approximately 1 centimeter per month. Other significant but less frequent features may be observed in hair. For example, morphological abnormalities may be present as a result of certain diseases or nutrient deficiencies. Also, the presence of fungal and nit infections can further link a hair specimen to a particular individual.

MICROSCOPIC EXAMINATION OF HAIR

A comparison microscope is an invaluable tool that allows the examiner to view the questioned and known hair together, side by side. Any variations in the microscopic characteristics will thus be readily observed. Because hair from any part of the body exhibits a range of characteristics, it is necessary to have an adequate number of known hairs that are representative of all its features when making a comparison.

Although the microscopic comparison of hairs has long been accepted as an appropriate approach for including and excluding questioned hairs against standard/reference hairs, many forensic scientists have long recognized that this approach is very subjective and is highly dependent on the skills and integrity of the analyst, as well as the hair morphology being examined. However, until the advent of DNA analysis, the forensic science community had no choice but to rely on the microscope to carry out hair comparisons.

Any lingering doubts about the necessity of augmenting microscopic hair examinations with DNA analysis evaporated with the publication of an FBI study describing significant error rates associated with microscopic comparison of

hairs.2 Hair evidence submitted to the FBI for DNA analysis between 1996 and 2000 was examined both microscopically and by DNA analysis.

Approximately 11 percent of the hairs (nine out of eighty) in which FBI hair examiners found a positive microscopic match between questioned and standard/reference hairs were found to be nonmatches when they were later subjected to DNA analysis. The course of events is clear: Microscopic hair comparisons must be regarded by police and courts as presumptive in nature, and all positive microscopic hair comparisons must be confirmed by DNA determinations.

QUESTIONS ABOUT HAIR EXAMINATION

A number of questions may be asked to further ascertain the present status of forensic hair examinations. The answers

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to these questions can be of great significance to the investigator working with hair evidence.

Can the Body Area from Which a Hair Originated Be Determined?

Normally it is easy to determine the body area from which a hair came. For example, scalp hairs generally show little diameter variation and have a more uniform distribution of pigment when compared to other body hairs. Pubic hairs are short and curly, with wide variations in shaft diameter, and usually have continuous medullae. Beard hairs are coarse, are normally triangular in cross-section, and have blunt tips acquired from cutting or shaving.

Can the Racial Origin of Hair Be Determined?

In many instances, the examiner can distinguish hair originating from members of different races; this is especially true of Caucasian and Negroid head hair. Negroid hairs are normally kinky, containing dense, unevenly distributed pigments. Caucasian hairs are usually straight or wavy, with very fine to coarse pigments that are more evenly distributed when compared to Negroid hair. Mongoloid hairs often have a dense pigment distribution, but they normally don’t exhibit the pigment clumping seen in Negroid hairs. Mongoloids also tend to have thicker hair shaft diameters when compared to the other two races.

Sometimes a cross-sectional examination of hair may help identify race. Cross-sections of hair from Caucasians are oval to round in shape, Mongoloid generally exhibit a round cross-sectional shape, and cross-sections of Negroid hair are flat to oval in shape. However, all of these observations are general, with many possible exceptions. The criminalist must approach the determination of race from hair with caution and a good deal of experience.

Can the Age and Sex of an Individual Be Determined from a Hair Sample?

The age of an individual cannot be learned from a hair examination with any degree of certainty except in the case of infant hairs, which are fine and short and have fine pigmentation. Although the presence of dye or bleach on the hair may offer some clue to sex, present hairstyles make these characteristics less valuable than they were in the past. The recovery of nuclear DNA either from tissue adhering to a hair or from the root structure of the hair will allow a determination of whether the hair originated from a male or female.

Is It Possible to Determine Whether Hair Was Forcibly Removed from the Body?

A microscopic examination of the hair root may establish whether the hair fell out or was pulled out of the skin. A hair root with follicular tissue (root sheath cells) adhering to it, as shown in Figure 13-8, indicates a hair that has been pulled out either by a person or by brushing or combing. Hair naturally falling off the body has a bulbous-shaped root free of any adhering tissue.

The absence of sheath cells cannot always be relied on for correctly judging whether hair has been forcibly pulled from the body. In some cases the root of a hair is devoid of any adhering tissue even when it has been pulled from the body. Apparently, an important consideration is how quickly the hair is pulled out of the head. Hairs pulled quickly from the

head are much more likely to have sheath cells compared to hairs that have been removed slowly from the scalp.3

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CASE FILES CENTRAL PARK JOGGER CASE REVISITED

On April 19, 1989, a young woman left her apartment around nine p.m. to jog in New York’s Central Park. Nearly five hours later, she was found comatose lying in a puddle of mud in the park. She had been raped, her skull was fractured, and she had lost 75 percent of her blood. When the woman recovered, she had no memory of what happened to her. The brutality of the crime sent shock waves through the city and seemed to fuel a national perception that crime was running rampant and unchecked through the streets of New York.

Already in custody at the station house of the Central Park Precinct was a group of 14- and 15-year-old boys who had been rounded up leaving the park earlier in the night by police who suspected that they had been involved in a series of random attacks. Over the next two days, four of the teenagers gave videotaped statements, which they later recanted, admitting to participating in the attack. Ultimately, five of the teenagers were charged with the crime.

Interestingly, none of the semen collected from the victim could be linked to any of the defendants. However, according to the testimony of a forensic analyst, two head hairs collected from the clothing of one of the defendants microscopically compared to those of the victim, and a third hair collected from the same defendant’s T-shirt microscopically compared to the victim’s pubic hair. Besides these three hairs, a fourth hair was found to be microscopically similar to the victim’s. This hair was recovered from the clothing of Steven Lopez, who was originally charged with rape but not prosecuted for the crime.

Hairs were the only pieces of physical evidence offered by the district attorney to directly link any of the teenagers to the crime. The hairs were cited by the district attorney as proof for the jury that the videotaped confessions of the teenagers were reliable. The five defendants were convicted and ultimately served from nine to thirteen years.

In August 1989, more than three months after the jogger attack, New York police arrested a man named Matias Reyes, who pleaded guilty to murdering a pregnant woman, raping three other women, and committing a robbery. For these crimes Reyes was sentenced to thirty-three years to life. In January 2002, Reyes also confessed to the Central Park attack. Follow-up tests revealed that Reyes’s DNA compared to semen recovered from the jogger’s body and her sock. Other DNA tests showed that the hairs offered into evidence at the original trial did not come from the victim and so could not be used to link the teenagers to the crime as the district attorney had argued. After an eleven-month reinvestigation of the original charges, a New York State Supreme Court judge dismissed all the convictions against the five teenage suspects in the Central Park jogger case.

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Courtesy AP Wide World Photos

FIGURE 13-8 Forcibly removed head hair with follicular tissue attached.

Are Efforts Being Made to Individualize Human Hair?

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As we will see in Chapter 15, forensic scientists routinely isolate and characterize individual variations in DNA. Forensic hair examiners can link human hair to a particular individual by characterizing the nuclear DNA in the hair root or in follicular tissue adhering to the root (see Figure 13-8). Recall that the follicular tag is the richest source of DNA associated with hair. In the absence of follicular tissue, an examiner must extract DNA from the hair root.

nuclear DNA

DNA that is present in the nucleus of a cell and that is inherited from both parents.

The growth phase of hair is a useful predictor of the likelihood of successfully typing DNA in human hair.4 Examiners have a higher success rate in extracting DNA from hair roots in the anagen phase or from anagen-phase hairs entering the catagen phase of growth. Telogen-phase hairs have an inadequate amount of DNA for typing. Because most hairs are naturally shed and are expected to be in the telogen stage, these observations do not portend well for hairs collected at crime scenes. However, some crime scenes are populated with forcibly removed hairs that are expected to be rich sources for nuclear DNA.

When a questioned hair does not have adhering tissue or a root structure amenable to isolation of nuclear DNA, there is an alternative source of information: mitochondrial DNA. Unlike the nuclear DNA described earlier, which is located in the nuclei of practically every cell in the body, mitochondrial DNA is found in cellular material outside the nucleus. Interestingly, unlike nuclear DNA, which is passed down from both parents, mitochondrial DNA is transmitted only from mother to child. Importantly, many more copies of mitochondrial DNA than nuclear DNA are located in the cells. For this reason, the success rate of finding and typing mitochondrial DNA is much greater from samples that have limited quantities of nuclear DNA, such as hair. Hairs 1 to 2 centimeters long can be subjected to mitochondrial analysis with extremely high odds of success. This subject is discussed in greater detail in Chapter 15.

mitochondrial DNA

DNA present in small structures (i.e., mitochondria) outside the nucleus of a cell. Mitochondria supply energy to the cell. This form of DNA is inherited maternally (from the mother).

Can DNA Individualize a Human Hair?

In some cases, the answer is yes. As we will learn in Chapter 15, nuclear DNA produces frequencies of occurrence as low as one in billions or trillions. On the other hand, mitochondrial DNA cannot individualize human hair. However, its diversity within the human population often permits the exclusion of a significant portion of a population as potential contributors of a hair sample. Ideally, the combination of a positive microscopic comparison and an association through nuclear or mitochondrial DNA analysis strongly links a questioned hair and standard/reference hairs. However, a word of caution: Mitochondrial DNA cannot distinguish microscopically similar hairs from individuals who are maternally related.

COLLECTION AND PRESERVATION OF HAIR EVIDENCE

When questioned hairs are submitted to a forensic laboratory for examination, they must always be accompanied by an adequate number of standard/reference samples from the victim of the crime and from individuals suspected of having deposited hair at the crime scene. We have learned that hair from different parts of the body varies significantly in its physical characteristics. Likewise, hair from any one area of the body can also have a wide range of characteristics. For this reason, the questioned and standard/reference hairs must come from the same area of the body; one cannot, for instance, compare head hair to pubic hair. It is also important that the collection of standard/ reference hair be carried

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out in a way that ensures a representative sampling of hair from any one area of the body.

CASE FILES

The murder of Ennis Cosby, son of entertainer Bill Cosby, at first appeared unsolvable. It was a random act. When his car tire went flat, Ennis pulled off the road and called a friend on his cellular phone to ask for assistance. Shortly thereafter, an assailant demanded money and, when Cosby didn’t respond quickly enough, shot him once in the temple. Acting on a tip from a friend of the assailant, police investigators later found a .38-caliber revolver wrapped in a blue cap miles from the crime scene. Mikail Markhasev was arrested and charged with murder.

Bill Cosby and his son Ennis Cosby.

Courtesy Andrea Mohin, The New York Times

At the trial, the district attorney introduced firearms evidence to show that the recovered gun had fired the bullet that killed Cosby. A single hair also recovered from the hat dramatically linked Markhasev to the crime: Los Angeles Police Department forensic analyst Harry Klann identified six DNA markers from the follicular tissue adhering to the hair root that matched Markhasev’s DNA. This particular DNA profile is found in 1 out of 15,500 members of the general population. On hearing all the evidence, the jury deliberated and convicted Markhasev of murder.

Forensic hair comparisons generally involve either head hair or pubic hair. Collecting fifty full-length hairs from all areas of the scalp normally ensures a representative sampling of head hair. Likewise, a minimum collection of twenty-four full-length pubic hairs should cover the range of characteristics present in this type of hair. In rape cases, care must first be taken to comb the pubic area with a clean comb to remove all loose foreign hair present before the victim is sampled for standard/reference hair. The comb should then be packaged in a separate envelope.

Because a hair may vary in color and other morphological features over its entire length, the entire hair is collected.

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This requirement is best accomplished by either pulling the hair out of the skin or clipping it at the skin line. During an autopsy, hair samples are routinely collected from victims of suspicious deaths. Because the autopsy may occur early in an investigation, the need for hair standard/reference samples may not always be apparent. However, one should never rule out the possible involvement of hair evidence in subsequent investigative findings. Failure to make this simple collection may result in complicated legal problems later.

Quick Review

• The hair shaft is composed of three layers called the cuticle, cortex, and medulla and is the part of a hair most intensely examined by the forensic scientist. • When comparing strands of hair, the criminalist is particularly interested in matching the color, length, and diameter. Other important features for comparing hair are the presence or absence of a medulla and the distribution, shape, and color intensity of pigment granules in the cortex. • The likelihood of successfully detecting DNA in hair roots is higher in hair being examined in its anagen or early growth phase than in its catagen or telogen phases. • The follicular tag, a translucent piece of tissue surrounding the hair’s shaft near the root, is a rich source of DNA associated with hair. Mitochondrial DNA can also be extracted from the hair shaft. • All positive microscopic hair comparisons must be confirmed by DNA analysis.

Forensic Examination of Fibers

Just as hair left at a crime scene can be used for identification, so can the fibers that compose fabrics and garments. Fibers may become important evidence in incidents that involve personal contact—such as homicide, assault, and sexual offenses—in which cross-transfers may occur between the clothing of suspect and victim. Similarly, the force of impact between a hit-and-run victim and a vehicle often leaves fibers, threads, or even whole pieces of clothing adhering to parts of the vehicle. Fibers may also become fixed in screens or on glass that is broken in the course of a breaking-and-entering attempt.

Regardless of where and under what conditions fibers are recovered, their ultimate value as forensic evidence depends on the criminalist’s ability to narrow their origin to a limited number of sources or even to a single source. Unfortunately, mass production of garments and fabrics has limited the value of fiber evidence in this respect, and only rarely do fibers recovered at a crime scene provide individual identification with a high degree of certainty.

TYPES OF FIBERS

For centuries, humans depended on fibers derived from natural sources such as plants and animals. However, early in the twentieth century, the first manufactured fiber—rayon—became a practical reality, followed in the 1920s by the introduction of cellulose acetate. Since the late 1930s, scientists have produced dozens of new fibers. In fact, there have been greater advances in the development of fibers, fabrics, finishes, and other textile-processing techniques since 1900 than in the preceding five thousand years of recorded history. Today, such varied items as clothing, carpeting, drapes, wigs, and even artificial turf attest to the predominant role that manufactured fibers have come to play in our culture and environment. When discussing forensic examination of fibers, it is convenient to classify them into two broad groups: natural and manufactured.

NATURAL FIBERS

Natural fibers are wholly derived from animal or plant sources. Natural fibers encountered in crime laboratory

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examinations come primarily from animals. These include hair coverings from such animals as sheep (wool), goats (mohair, cashmere), camels, fiamas, alpacas, and vicuñas. Fur fibers include those obtained from animals such as mink, rabbit, beaver, and muskrat.

natural fibers

Fibers derived entirely from animal or plant sources.

The forensic examination of animal fibers uses the same procedures discussed in the previous section for the forensic examination of animal hairs. The identification and comparison of such fibers relies solely on a microscopic examination of color and morphological characteristics. Again, a sufficient number of standard/reference specimens must be examined to establish the range of fiber characteristics that make up the suspect fabric.

FIGURE 13-9 Photomicrograph of cottonfiber (450×).

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By far the most prevalent plant fiber is cotton. The wide use of undyed white cotton fibers in clothing and other fabrics has made its evidential value almost meaningless, but the presence of dyed cotton in a combination of colors has, in some cases, enhanced its evidential significance. The microscopic view of cotton fiber shown in Figure 13-9 reveals its most distinguishing feature—its ribbonlike shape with twists at irregular intervals.

MANUFACTURED FIBERS

Beginning with the introduction of rayon in 1911 and the development of nylon in 1939, manufactured fibers have increasingly replaced natural fibers in garments and fabrics. Such fibers are marketed under hundreds of trade names. To reduce consumer confusion, the US Federal Trade Commission has approved “generic” or family names for the grouping of all manufactured fibers. Many of these generic classes are produced by several manufacturers and are sold under a confusing variety of trade names. For example, in the United States, polyesters are marketed under names that

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include Dacron, Fortrel, and Kodel. In England, polyesters are called Terylene. Table 13.1 lists major generic fibers, along with common trade names and their characteristics and applications.

manufactured fibers

Fibers derived from either natural or synthetic polymers.

The first machine-made fibers were manufactured from raw materials derived from cotton or wood pulp, and these are still being made. The raw materials are processed, and pure cellulose is extracted from them. Depending on the type of fiber desired, the cellulose may be chemically treated and dissolved in an appropriate solvent before it is forced through the small holes of a spinning jet, or spinneret, to produce the fiber. Fibers manufactured from natural raw materials in this manner are classified as regenerated fibers and commonly include rayon, acetate, and triacetate, all of which are produced from regenerated cellulose.

Most of the fibers currently manufactured are produced solely from synthetic chemicals and are therefore classified as synthetic fibers. These include nylons, polyesters, and acrylics. The creation of synthetic fibers became a reality only when scientists developed a method of synthesizing long-chained molecules called polymers.

In 1930, chemists discovered an unusual characteristic of one of the polymers under investigation. When a glass rod in contact with viscous material in a beaker was slowly pulled away, the substance adhered to the rod and formed a fine filament that hardened as soon as it entered the cool air. Furthermore, the cold filaments could be stretched several times their extended length to produce a flexible, strong, and attractive fiber. This first synthetic fiber was improved and then marketed as nylon. Since then, fiber chemists have successfully synthesized new polymers and have developed more efficient methods for manufacturing them. These efforts have produced a multitude of synthetic fibers.

IDENTIFICATION AND COMPARISON OF MANUFACTURED FIBERS

The evidential value of fibers lies in the criminalist’s ability to trace their origin. Obviously, if the examiner is presented with fabrics that can be exactly fitted together at their torn edges, the fabrics must be of common origin.

More often, however, the criminalist obtains a limited number of fibers for identification and comparison. Generally, in these situations obtaining a physical match is unlikely, and the examiner must resort to a side-by-side comparison of the standard/reference and crime-scene fibers.

MICROSCOPIC EXAMINATION OF FIBERS

The first and most important step in the examination is a microscopic comparison for color and diameter using a comparison microscope. Unless these two characteristics agree, there is little reason to suspect a match. Other morphological features that may aid in the comparison are lengthwise striations (lined markings) on the surface of some fibers and the pitting of the fiber’s surface with delustering particles (usually titanium dioxide) added in the manufacturing process to reduce shine (see Figure 13-10).

TABLE 13.1 Major Generic Fibers

MAJOR CHARACTERISTICS MAJOR DOMESTIC AND INDUSTRIAL USES

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GENERIC FIBER

Acetate

• Luxurious feel and appearance • Wide range of colors and lusters • Excellent drapability and softness • Relatively fast-drying • Shrink-, moth-, and mildew-resistant

Apparel: Blouses, dresses, foundation garments, lingerie, linings, shirts, slacks, sportswear

Fabrics: Brocade, crepe, double knits, faille, knitted jerseys, lace, satin, taffeta, tricot

Home Furnishings: Draperies, upholstery

Other: Cigarette filters, fiberfill for pillows, quilted products

Acrylic

• Soft and warm • Wool-like • Retainsshape • Resilient • Quick-drying • Resistanttomoths, sunlight, oil, and chemicals

Apparel: Dresses, infant wear, knitted garments, skiwear, socks, sportswear, sweaters

Fabrics: Fleece and pile fabrics, face fabrics in bonded fabrics, simulated furs, jerseys

Home Furnishings: Blankets, carpets, draperies, upholstery

Other: Auto tops, awnings, hand-knitting and craft yarns, industrial and geotextile fabrics

Aramid

• Does not melt • Highly flame-resistant • Great strength • Great resistance to stretch • Maintains shape and form at high temperatures

Hot-gas filtration fabrics, protective clothing, military helmets, protective vests, structural composites for aircraft and boats, sailcloth, tires, ropes and cables, mechanical rubber goods, marine and sporting goods

Bicomponent

• Thermal bonding • Self-bulking • Very fine fibers • Unique cross-sections • The functionality of special polymers or additives at reduced cost

Uniform distribution of adhesive; fiber remains a part of structure and adds integrity; customized sheath materials to bond various materials; wide range of bonding temperatures; cleaner, environmentally friendly (no effluent); recyclable; lamination/molding/densification of composites

Lyocell

• Soft, strong, absorbent • Good dyeability • Fibrillates during wet processing to produce special textures

Dresses, slacks, and coats

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Melamine

• White and dyeable • Flame resistance and low thermal conductivity • High-heat dimensional stability • Processable on standard textile equipment

Fire-Blocking Fabrics: Aircraft seating, fire blockers for upholstered furniture in high-risk occupancies (e.g., to meet California TB 133 requirements)

Protective Clothing: Firefighters’ turnout gear, insulating thermal liners, knit hoods, molten metal splash apparel, heat-resistant gloves

Filter Media: High-capacity, high-efficiency, high-temperature baghouse air filters

Modacrylic

• Soft • Resilient • Abrasion- and flame- resistant • Quick-drying • Resists acids and alkalies • Retains shape

Apparel: Deep-pile coats, trims, linings, simulated fur, wigs and hairpieces

Fabrics: Fleece fabrics, industrial fabrics, knit-pile fabric backings, nonwoven fabrics

Home Furnishings: Awnings, blankets, carpets, flame-resistant draperies and curtains, scatter rugs

Other: Filters, paint rollers, stuffed toys

Nylon

• Exceptionally strong • Supple • Abrasion-resistant • Lustrous • Easy to wash • Resists damage from oil and many chemicals • Resilient • Low in moisture absorbency

Apparel: Blouses, dresses, foundation garments, hosiery, lingerie and underwear, raincoats, ski and snow apparel, suits, windbreakers

Home Furnishings: Bedspreads, carpets, draperies, curtains, upholstery

Other: Air hoses, conveyor and seat belts, parachutes, racket strings, ropes and nets, sleeping bags, tarpaulins, tents, thread, tire cord, geotextiles

Olefin

• Unique wicking properties that make it very comfortable • Abrasion-resistant • Quick-drying • Resistant to deterioration from chemicals, mildew, perspiration, rot, and weather • Sensitive to heat • Soil-resistant • Strong; very lightweight • Excellent colorfastness

Apparel: Pantyhose, underwear, knitted sports shirts, men’s half-hose, men’s knitted sportswear, sweaters

Home Furnishings: Carpet and carpet backing, slipcovers, upholstery

Other: Dye nets, filter fabrics, laundry bags, sandbags, geotextiles, automotive interiors, cordage, doll hair, industrial sewing thread

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Polyester

• Strong • Resistant to stretching and shrinking • Resistant to most chemicals • Quick-drying • Crisp and resilient when wet or dry • Wrinkle- and abrasion- resistant • Retains heat-set pleats and creases • Easy to wash

Apparel: Blouses, shirts, career apparel, children’s wear, dresses, half-hose, insulated garments, ties, lingerie and underwear, permanent press garments, slacks, suits

Home Furnishings: Carpets, curtains, draperies, sheets and pillowcases

Other: Fiberfill for various products, fire hoses, power belting, ropes and nets, tire cord, sail, V-belts

PBI

• Extremely flame-resistant • Outstanding comfort factor combined with thermal and chemical stability properties • Will not burn or melt • Low shrinkage when exposed to flame

Suitable for high-performance protective apparel such as firefighters’ turnout coats, astronaut space suits, and applications in which fire resistance is important

Rayon

• Highly absorbent • Soft and comfortable • Easy to dye • Versatile • Good drapability

Apparel: Blouses, coats, dresses, jackets, lingerie, linings, millinery, rainwear, slacks, sports shirts, sportswear, suits, ties, work clothes

Home Furnishings: Bedspreads, blankets, carpets, curtains, draperies, sheets, slipcovers, tablecloths, upholstery

Other: Industrial products, medical-surgical products, nonwoven products, tire cord

Spandex

• Can be stretched 500 percent without breaking • Can be stretched repeatedly and recover original length • Lightweight • Stronger and more durable than rubber • Resistant to body oils

Apparel (articles in which stretch is desired): Athletic apparel, bathing suits, delicate laces, foundation garments, golf jackets, ski pants, slacks, support and surgical hose

Source: American Fiber Manufacturers Assoc. Inc., Washington, DC, www.fingersource.com Reprinted by permission.

FIGURE 13-10 Photomicrographs of synthetic fibers: (a) cellulose

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triacetate (450×) and (b) olefin fiber embedded with titanium dioxide particles (450×).

FIGURE 13-11 Cross-sectional shapes of fibers.

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The cross-sectional shape of a fiber may also help characterize the fiber (see Figure 13-11).5 In the early 1880s, Wayne Williams was charged and tried for the murder of two individuals in the Atlanta, Georgia, region. During the eight-week trial, evidence linking Williams to those murders and to the murder of ten other individuals was introduced. An essential part of the government’s case was the numerous fibers linking Williams to the murders. Unusually shaped yellow-green fibers discovered on a number of the murder victims were linked to a carpet in the Williams home. This fiber was a key element in proving Williams’s guilt. A photomicrograph of this unusually shaped fiber is shown in Figure 13-12.

FIGURE 13-12 A scanning electron photomicrograph of the cross-section of a nylon fiber removed from a sheet used to transport the body of a murder victim. The fiber, associated with a carpet in Wayne Williams’s home, was manufactured in 1971 in relatively small quantities.

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Courtesy Federal Bureau of Investigation, Washington, DC

Although two fibers may seem to have the same color when viewed under the microscope, compositional differences may actually exist in the dyes that were applied to them during their manufacture. In fact, most textile fibers are impregnated with a mixture of dyes selected to obtain a desired shade or color. The significance of a fiber comparison is enhanced when the forensic examiner can show that the questioned and standard/reference fibers have the same dye composition.

ANALYTICAL TECHNIQUES USED IN FIBER EXAMINATION

In Chapter 11, we saw how a chemist can use selective absorption of light by materials to characterize them. In particular, light in the ultraviolet, visible, and infrared regions of the electromagnetic spectrum is most helpful for this purpose. Unfortunately, in the past, forensic chemists were unable to take full advantage of the capabilities of spectrophotometry for examining trace evidence because most spectrophotometers are not well suited for examining the very small particles frequently encountered as evidence. Recently, linking the microscope to a computerized spectrophotometer has added a new dimension to its capability. This combination has given rise to a new instrument called the microspectrophotometer. In many respects, this is an ideal marriage from the forensic scientist’s viewpoint.

The visible-light microspectrophotometer is a convenient way for analysts to compare the colors of fibers through spectral patterns. This technique is not limited by sample size; a fiber as small as 1 millimeter long or less can be examined by this type of microscope. The examination is nondestructive and is carried out on fibers simply mounted on a microscope slide.

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CHEMICAL COMPOSITION

Before the forensic scientist can reach a conclusion that two or more fibers compare, it must be shown that the fibers in question have the same chemical composition. In this respect, tests are performed to confirm that all of the fibers involved belong to the same broad generic class. Additionally, the comparison will be substantially enhanced if it can be demonstrated that all of the fibers belong to the same subclassification within their generic class. For example, at least four types of nylon are available in commercial and consumer markets, including nylon 6, nylon 6-10, nylon 11, and nylon 6-6. Although all types of nylon have many properties in common, each may differ in physical shape, appearance, and dyeability because of modifications in their basic chemical structure.

CLOSER ANALYSIS THE MICROSPECTROPHOTOMETER

With the development of the microspectrophotometer, a forensic analyst can view a particle under a microscope while a beam of light is directed at the particle to obtain its absorption spectrum. Depending on the type of light employed, an examiner can acquire either a visible or an infrared (IR) spectral pattern of the substance being viewed under the microscope. The obvious advantage of this approach is that it provides added information to characterize trace quantities of evidence. A microspectrophotometer designed to measure the uptake of visible light by materials is shown here.

Visual comparison of color is usually one of the first steps in examining paint, fiber, and ink evidence. Such comparisons are easily obtained using a comparison microscope. A forensic scientist can use the microspectrophotometer to compare the color of materials visually while plotting an absorption spectrum for each item under examination. This displays the exact wavelengths at which each item absorbs in the visible-light spectrum. Occasionally, colors that appear similar by visual examination show significant differences in their absorption spectra.

Another emerging technique in forensic science is the use of the IR microspectrophotometer to examine fibers and paints. The “fingerprint” IR spectrum (see Figure 1 and 2 in the Case File on page 338) is unique for each chemical substance. Therefore, obtaining such a spectrum from either a fiber or a paint chip allows the analyst to better identify and compare the type of chemicals from which these materials are manufactured. With a microspectrophotometer, a forensic analyst can view a substance through the microscope and at the same time have the instrument plot the infrared absorption spectrum for that material.

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A visible-light microspectrophotometer.

Courtesy CRAIC Technologies Inc., Altadena, CA, www.microspectra.com

Textile chemists have devised numerous tests for determining the class of a fiber. However, unlike the textile chemist, the criminalist frequently does not have the luxury of a substantial quantity of the fabric to work with and must therefore select tests that will yield the most information with the least amount of material. Only a single fiber may be available for analysis, and often this may amount to no more than a minute strand recovered, for example, from a fingernail scraping from a homicide or rape victim.

INFRARED ABSORPTION

The polymers that compose a manufactured fiber, like any organic substance, selectively absorb infrared light in a characteristic pattern. Infrared spectrophotometry thus provides a rapid and reliable method for identifying the generic class, and in some cases the subclass, of a fiber. The infrared microspectrophotometer combines a microscope with an infrared spectrophotometer. Such a combination makes possible the infrared analysis of a small, single-strand fiber while it is being viewed under a microscope.

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SIGNIFICANCE OF FIBER EVIDENCE

Once a fiber match has been determined, the question of the significance of such a finding is bound to be raised. In reality, no analytical technique permits the criminalist to link a fiber strand definitively to any single garment. Furthermore, except in the most unusual circumstances, no statistical databases are available for determining the probability of a fiber’s origin. Considering the mass distribution of synthetic fibers and the constantly changing fashion tastes of our society, it is highly unlikely that such data will be available in the foreseeable future.

Despite these limitations, an investigator should not discount or minimize the significance of a fiber association. An enormous variety of fibers exists in our society. By simply looking at the random individuals we meet every day, we can see how unlikely it is to find two people wearing identically colored fabrics (with the exception of blue denims or white cottons). There are thousands of different-colored fibers in our environment. Combine this with the fact that forensic scientists compare not only the color of fibers but also their size, shape, microscopic appearance, chemical composition, and dye content, and one can now begin to appreciate how unlikely it is to find two indistinguishable colored fibers on two randomly selected sources.

Furthermore, the significance of a fiber association increases dramatically when the analyst can link two or more distinctly different fibers to the same object. Likewise, the associative value of fiber evidence is dramatically enhanced if it is accompanied by other types of physical evidence linking a person or object to a crime. As with most class evidence, the significance of a fiber comparison is dictated by the circumstances of the case; by the location, number, and nature of the fibers examined; and, most important, by the judgment of an experienced examiner.

Collection and Preservation of Fiber Evidence

As criminal investigators have become more aware of the potential contribution of trace physical evidence to the success of their investigations, they have placed greater emphasis on conducting thorough crime-scene searches for evidence of forensic value. Their skill and determination at carrying out these tasks is tested in the collection of fiber- related evidence. Fiber evidence can be associated with virtually any type of crime. It usually cannot be seen with the naked eye and thus can be easily overlooked by someone not specifically searching for it.

An investigator committed to optimizing the laboratory’s chances for locating minute strands of fibers identifies and preserves potential “carriers” of fiber evidence. Relevant articles of clothing should be packaged carefully in paper bags. Each article must be placed in a separate bag to avoid cross-contamination of evidence. Scrupulous care must be taken to prevent articles of clothing from different people or from different locations from coming into contact. Such articles must not even be placed on the same surface prior to packaging. Likewise, carpets, rugs, and bedding are to be folded carefully to protect areas suspected of containing fibers. Car seats should be carefully covered with polyethylene sheets to protect fiber evidence, and knife blades should be covered to protect adhering fibers. If a body is thought to have been wrapped at one time in a blanket or carpet, adhesive tape lifts of exposed body areas may reveal fiber strands.

Occasionally the field investigator may need to remove a fiber from an object, particularly if loosely adhering fibrous material may be lost in transit to the laboratory. These fibers must be removed with a clean forceps and placed in a small sheet of paper, which, after folding and labeling, should be placed inside another container. Again, scrupulous care must be taken to prevent contact between fibers collected from different objects or from different locations.

In the laboratory, the search for fiber evidence on clothing and other relevant objects, as well as in debris, is time consuming and tedious and will test the skill and patience of the examiner. The crime-scene investigator can manage this task by collecting only relevant items for examination—pinpointing areas where a likely transfer of fiber evidence

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occurred and then ensuring the proper collection and preservation of these materials.

CASE FILES FATAL VISION REVISITED

Dr. Jeffrey MacDonald, pictured here, was convicted in 1979 of murdering his wife and two young daughters. The events surrounding the crime and the subsequent trial were recounted in Joe McGinniss’s best-selling book Fatal Vision. The focus of Dr. MacDonald’s defense was that intruders entered his home and committed these violent acts. Eleven years after this conviction, Dr. MacDonald’s attorneys filed a petition for a new trial, claiming the existence of “critical” new evidence.

The defense asserted that wig fibers found on a hairbrush in the MacDonald residence were evidence that an intruder dressed in a wig entered the MacDonald home on the day of the murder. Subsequent examination of this claim by the FBI Laboratory focused on a blond fall (a type of artificial hair extension) frequently worn by Dr. MacDonald’s wife. Fibers removed from the fall were shown to clearly match fibers on the hairbrush. The examination included the use of infrared microspectrophotometry to demonstrate that the suspect wig fibers were chemically identical to fibers found in the composition of Mrs. MacDonald’s fall (see Figure 1). Hence, although wig fibers were found at the crime scene, the source of these fibers could be accounted for: Mrs. MacDonald’s fall.

Jeffrey MacDonald in 1995 at Sheridan, Oregon, Federal Correctional Institution.

Courtesy AP Wide World Photos

Another piece of evidence cited by Dr. MacDonald’s lawyers was a bluish-black woolen fiber found on the body of Mrs. MacDonald. They claimed that this fiber compared to a bluish-black woolen fiber recovered from the club used to assault her. These wool fibers were central to Dr. MacDonald’s defense that the “intruders” wore dark-colored clothing. Initial examination showed that the fibers were microscopically indistinguishable. However, the FBI also compared the two wool fibers by visible-light microspectrophotometry. Comparison of their spectra clearly showed that their dye compositions differed, providing no evidence of outside intruders (see Figure 2). Ultimately, the US Supreme Court denied the merits of Jeffrey MacDonald’s petition for a new trial.

Source: Based on information contained in B. M. Murtagh and M. P. Malone, “Fatal Vision Revisited,” Police Chief (June 1993): 15.

FIGURE 1 A fiber comparison made with an infrared spectrophotometer.

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The infrared spectrum of a fiber from Mrs. MacDonald’s fall compares to a fiber recovered from a hairbrush in the MacDonald home. These fibers were identified as modacrylics, the most common type of synthetic fiber used in the manufacture of human hair goods.

Courtesy SA Michael Malone, FBI Laboratory, Washington, DC

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